Danish Patent No. 149238 discloses a zero-flux current transformer for measuring the current for an electromagnet in a particle accelerator. As very heavy current intensities are involved, viz. several hundred amperes, the main current is advantageously converted into a comparatively weaker measuring current which is easy to handle, and which is passed through a measuring resistor. Then the voltage drop across the measuring resistor is used as a measurement representing the intensity of the main current.
The known zero-flux current transformers are formed by a second harmonic magnetic modulator in combination with a magnetic integrator. In principle a magnetic integrator includes an annular core of a ferromagnetic material and is provided with a primary winding, a secondary winding and a sensor winding. The sensor winding communicates with the inlet terminals of an amplifier driving the secondary current through a measuring resistor. Then an alteration of the magnetic flux in the annular core induces a voltage in the sensor winding, and this voltage affects the amplifier in such a manner that said magnetic flux generates a compensating current which counteracts the alteration of the flux caused by the primary current. Thus the magnetomotive forces generated by the current through the primary winding are equalized by the magnetomotive forces generated by the current through the secondary winding in such a manner that a specific relationship exists between the current intensity in the primary winding and the current intensity in the secondary winding.
However, the magnetic integrator cannot handle DC and very low frequencies, and accordingly it is necessary to include a separate circuit for this function. Such a circuit is formed by a magnetic modulator including two identically coated annular cores and a driver circuit. The annular cores are driven in saturation, and at a balance/“zero flux” the current curves are symmetrical relative to zero with the result that the contents of equal harmonics are zero. Then it is possible to use either a direct, symmetrical detection by means of a window comparator/Schmitt trigger or a second harmonic detector using synchronous rectification of the current signal presenting the double frequency of the modulation signal. In both cases an output signal is obtained which is zero at a balancing between the primary and the secondary ampere winding number. An unbalance between the ampere winding numbers results in a voltage of an amplitude and a polarity which depend on the extent and polarity of said unbalance. One core suffices for carrying out the latter detector function, but it is necessary to involve two cores in antiphase in order to prevent the compensating circuit from suppressing the detection and in order to prevent the modulation signal from disturbing the magnetic integrator through a magnetic coupling.
However, the known circuits are encumbered with the draw-back that it is not possible to compensate for alternating currents of a particularly high frequency. In addition, undesirable resonances are generated which can have a destroying effect.